CMP polishing slurry dewatering and reconstitution

ABSTRACT

A dry particulate solids composition is provided which may be reconstituted into a chemical-mechanical polishing slurry.

This application is a divisional of Ser. No. 09/413,083 filed Oct. 6,1999 U.S. Pat. No. 6,241,586 which claims the benefit of U.S.Provisional Application Ser. No. 60/103,283 filed Oct. 6, 1998.

BACKGROUND OF THE INVENTION

Polishing formulations for the chemical mechanical planarization (CMP)of integrated circuit wafers, as well as polishing compounds for hightechnology optical components typically consist of an aqueous dispersionof solids (the abrasives) combined with a variety of chemicalconstituents. Such commercial materials typically are sold in severalparts, one containing the abrasive component in a concentrated form andthe other part containing the chemical component(s) in a concentratedform. As the technology of CMP polishing has become more critical,complex chemical and abrasive systems have been developed. These aqueousformulations must exhibit long shelf life, and good stability so thatthe materials do not change while in storage, thus becoming useless fortheir intended purpose. For example, if a dispersion of solids is storedand the solids settle in the container to a hard or difficult to mixsediment, the dispersion is no longer readily usable by the customer.

Many attempts to make one component systems in which the chemical aswell as the abrasive components are mixed in one liquid system haveproven problematic. The problems occur because the presence of thechemical constituents is rarely conducive to forming stable soliddispersions, consequently the solids settle to the bottom of thecontainer and require subsequent redispersion prior to use. Thispresents many problems to the end user. If the solids have settled intoa difficult to redisperse sludge, the chances of utilizing all of thesettled material are not good because it will stick to the container.Furthermore, shaking or stirring a large container such as a drum ortote is impractical.

The ideal concept would be to make a slurry exactly as the final enduser would use it, and then “freeze it in time” so that it will beexactly as the end user wants it, three or even six months after theslurry is made. Since the main reason for instability is due to theaqueous dispersions that these slurries are sold as, it would bereasonable to assume that if one could eliminate the “aqueous” part ofthe system, the stability issues for both the chemical and abrasiveconstituents would be minimized.

SUMMARY OF THE INVENTION

A dry particulate solids composition comprised of a chemical-mechanicalpolishing slurry comprising submicron abrasive particles is providedwhich has had substantially all water removed therefrom and which may bereconstituted into a chemical-mechanical polishing slurry ready for use.

A method for chemical-mechanical polishing is provided comprising thesteps of:

a) providing a polishing pad;

b) introducing a polishing fluid comprising submicron abrasive particlesbetween said pad and a workpiece;

c) producing relative motion between said pad and said workpiece;

wherein said polishing fluid has been reconstituted from a dryparticulate solids composition comprised of a chemical-mechanicalpolishing slurry comprising submicron abrasive particles which has hadsubstantially all water removed therefrom.

In addition to submicron abrasive particles, the dry particulate solidscomposition of this invention may comprise an oxidizing agent, acomplexing agent, a surface passivating agent, a surfactant, adispersant, or any other type of compound used in slurries forchemical-mechanical polishing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a typical spray dryer.

FIG. 2 is a schematic of a typical slurry reconstitution.

FIG. 3 is a comparison of particle size distribution for three slurriesof Example 1.

DESCRIPTION OF THE INVENTION

This invention provides a means of preserving the exact abrasive andchemical ratios without the destabilizing and aging effects of aqueousdispersions. The abrasive and chemicals are mixed together as if theywere being prepared for the end user's immediate use, or they areprepared as a concentrate with the ratios of the components being allrelated by a common multiple. Immediately after the polishing slurry ismanufactured, it is dewatered by spray drying, freeze drying, or anynumber of drying or dewatering methods that exist. The resulting driedmaterial, usually in the form of a flowable powder, can be packaged inconventional bags and stored indefinitely without degradation.

A further advantage brought about by this invention is that the weightof the product is reduced by as much as 70%-80%, which is a considerablecost savings when one considers shipment of liquid, aqueous slurryworldwide.

When the slurry is ready to be used, the end user may reconstitute thedried material by adding the required amount of water and dispersingwith a high shear disperser. The resulting slurry is filtered and isready for use in the CMP polishing operation.

The resulting polishing slurry does not acquire unwanted aggregates asone would expect. This is thought to be due to the complete and thoroughdispersed state that the slurry is brought to just prior to drying. Asthe colloidially dispersed mixture of components dries, the abrasivegrains are surrounded by the uniform presence of the drying dissolvedsalts, which in effect protect the abrasive particles from agglomeratingwith each other. As the salts are solvated during the reconstitutionprocess, the abrasive particles are released in essentially the samestate they existed prior to being dried.

Typical submicron abrasives used in chemical-mechanical polishingslurries are oxides such as alumina, silica, ceria, titania, germania,zirconia, and the like. Generally abrasive particles are used inslurries for CMP at about 1% to about 15% by weight. Preferred arealumina, silica, ceria, titania, or mixtures thereof at about 3% to 10%by weight.

Types of chemicals useful in CMP operations are oxidizing agents,chemical etchants, dispersing agents, surfactants, complexing agents,silica rate suppressing agents, passivating agents, silica protectingagents, buffers, and inhibitors, all of which may be present in thedried slurries of the present invention.

Commonly hydroxides, such as potassium hydroxide, ammonium hydroxide,and sodium hydroxide, and amines have been used as dispersing agents forCMP slurry abrasives. It has been found that a class of compounds knownas amino alcohols may also be useful.

An oxidizing agent is usually a component of a chemical-mechanicalpolishing slurry to oxidize a metal layer to its corresponding oxide,such as oxidizing tungsten to tungsten oxide. The layer is mechanicallypolished to remove the tungsten oxide from the layer. Although a widernage of oxidizing components may be used, preferred components includeoxidizing metal salts, oxidizing metal complexes, iron salts such asnitrates, sulfates, EDTA, citrates, potassium ferricyanide and the like,aluminum salts, sodium salts, potassium salts, ammonium salts,quaternary ammonium salts, phosphonium salts, peroxides, chlorates,perchlorates, permanganates, persulfates, iodates, and mixtures thereof.Typically, the oxidizing component is present in the slurry in an amountsufficient to ensure rapid oxidation of a metal layer while balancingthe mechanical and chemical polishing components of the slurry.Oxidizing agents are typically present in a chemical-mechanical slurryfrom about 0.5% to 15% by weight, and preferably in a range from about1% to 7% by weight.

Compositions of this invention may optionally further comprise compoundswhich act as complexing agents or chelating agents for SiO₂. These aredescribed in great detail in U.S. Pat. No. 5,391,258 and U.S. Pat. No.5,476,606. These compounds must have at least two acid groups present inthe structure which can affect complexation to the silica. Acid speciesare defined as those functional groups having a dissociable proton.These include, but are not limited to, carboxyl, hydroxyl, sulfo andphospho groups. Carboxyl and hydroxyl groups are preferred as these arepresent in the widest variety of effective species. Particularlyeffective are structures which possess two or more carboxyl groups withhydroxyl groups in an alpha position, such as straight chain mono- anddi-carboxylic acids and salts including, for example, malic acid andmalates, tartaric acid and tartarates and gluconic acid and gluconates.Also effective are tri- and polycarboxylic acids and salts withsecondary or tertiary hydroxyl groups in an alpha position relative to acarboxyl group such as citric acid and citrates. Also effective arecompounds containing a benzene ring such as ortho di- andpolyhydroxybenzoic acids and acid salts, phthalic acid and acid salts,pyrocatecol, pyrogallol, gallic acid and gallates and tannic acid andtannates. These complexing agents may be used in slurries for CMP atabout 0.1% to about 7% by weight. Preferably they are used at about 2%to about 4% by weight.

In addition the slurry composition may include a chemical etchant whichis substantially free of metal ions. Typical etchants include persulfatesalts, nitrate salts, sulfate salts, phosphate salts, citrate salts,oxalate salts, mixtures thereof, and the like. Preferably, the etchantis a non-metallic persulfate salt such as ammonium persulfate. Theetchant facilitates the solubilization of the metal where the chemicalmechanical polishing is taking place, thus allowing the metal to bedissolved in the aqueous dispersion. These chemicals are generally foundin CMP slurries at about 1% to 10% by weight. Preferably they are usedat about 2% to 7% by weight.

Often a chemical-mechanical polishing slurry will comprise a corrosioninhibitor which is substantially free of metal ions. Suitable corrosioninhibitors include benzotriazole (BTA), mercaptobenzothiazole (MBT), andother corrosion inhibitors typically used with metals such as copperwhich do not adversely affect the properties of the slurry. Typicallycorrosion inhibitors are present in chemical-mechanical polishingslurries at less than about 1% by weight. Preferably they are present inthe range from about 0.05% to 0.6% by weight.

The first step in preparing dried slurry materials is to determine theexact ratio of all of the components in a polishing slurry as they existat the time of use. Let us assume that the slurry we wish toreconstitute is a 5% solid abrasive and 7% chemical constituents, andthat there are two chemical constituents. The total composition of aready to use formulation may look similar to the following example:

Solid Abrasive 5% Chemical A 3% Chemical B 4% Water 88% 

After the water has been removed, the solids and dissolved solids willbe left in the ratio of 5/3/4. In spray drying it is generally good tohave the highest solids content possible, to reduce the amount of waterthat has to be evaporated, thus reducing the cost and increasing thethroughput. Thus, when making a concentrate for drying, one should usethe constituents in the ratio of 5/3/4 times some factor. This factor istypically determined by a solubility limit of one of the components, ora maximum solids loading limitation. Let us assume that Chemical B isonly soluble to the maximum extent of 10%, then in order to make acompletely homogeneous dispersion where all of the chemical componentsare dissolved, the maximum concentration factor would be 10%/4% or 2.5.Thus, the concentration of the components of the pre-dried slurry isdetermined by multiplying each component except the water by the factor2.5. The final concentration for the drying slurry is:

Solid Abrasive 2.5 × 5 = 12.5% Chemical A 2.5 × 3 = 7.5%  Chemical B 2.5× 4 = 10.0% Water Balance 70.0%

This formulation when dried will yield a dried material with the threeactive constituents having the ratio of 5/3/4 as in the ready to useslurry but by concentrating the spray dryer starting material, the spraydrying operation is more efficient.

The drying technique can be one of many including spray drying, freezedrying, flash drying, vacuum drying, heated pan or conveyor drying, etc.The preferred one chosen for this invention is spray drying because itoffers the most easily controlled final product with respect to particlesize, % moisture, and reproducibility of product.

A spray dryer utilizes the combination of hot air circulating in a largetank into which a solid/liquid slurry is sprayed, either by rotaryatomization or high pressure aspiration, into the heated air wherein thewater content is almost immediately removed and a water-free solidparticle is formed. As the particle progresses further in the airstream, the particle is further dried until it reaches its exit point inthe chamber and is captured by a cyclone or collection container. Aschematic of a typical spray dryer is shown in FIG. 1. In many dryersthe coarser material is captured in a product collection container andthe fines are collected in a cyclone separator. In the case of thisinvention, both collection methods are satisfactory as the size of thedried particle is not an issue.

The dried product is then transported to a storage hopper wherein enoughspray dried material is collected to make a full lot. The collectedproduct is then dry blended in a large double cone blender, or similartype, so that the dried powder lot is completely homogeneous. Byblending a large lot dry, the size of the individual blended lot can bemuch larger than its liquid equivalent because the weight and volume ofthe water has been removed from the concentrate. A large homogeneous lotsize is particularly important for the semiconductor industry, whereeach new lot of material must be checked prior to use.

After blending, the powder is packaged in any suitable type ofcontainer, such as a plastic lined fiber drum, or a plastic lined paperbag. It is desirable for the container size to be equivalent to theamount of dried material that is likely to be used in each polishingcampaign after reconstitution.

Reconstitution of the dried material may be carried out in a mixing unitas shown in FIG. 2. The equipment consists of a suitable means ofintroducing the measured amount of water and the powdered slurry. Themixer is a high speed rotor stator design capable of providing very highshear. After the reconstituted product has been mixed adequately(generally determined by how much energy per gallon has been expended onthe product) the product is pumped from the reconstituting equipmentthrough a submicron filter system and into a day tank. From the daytank, the product is recirculated and used by the end user in hispolishing application.

It is in this last operation that the novelty and benefit of thisinvention is demonstrated. If the slurry is a two component system, thecomponent containing the solids typically must be premixed toredistribute the solids in the container prior to dispensing into theday tank. If this is not done, the correct % solids in the final mixturewill be incorrect. Similarly, the second component should be pre-mixedto homogenize the solution prior to adding to the day tank. Both ofthese components generally require some type of metering, either byweight or volume, in addition to the metering of water in someconcentrated systems. This mixture must then be filtered to remove anycontamination from the operation.

In the case of this invention, no pre-mixing is required, because nosettling or stability issues are involved. The powder is merely weighed(or pre-weighed for each size day tank), poured or educted into a tankcontaining the correct volume of water, and mixed. Upon completion ofmixing, the final product is pumped through a filter to the day tank. Insome cases, the day tank could be utilized as the mixing and dispersingvessel.

EXAMPLE 1

A slurry formulated for polishing copper in integrated circuitapplications, Met410, was used as one example of this invention. Thisslurry typically has a very short shelf life as a one component slurry,and only a short (several weeks) as a two component slurry becauseseveral of the components react with one another. Another component islight sensitive in the liquid form. The drying of this slurry is furthercomplicated by the fact that several of the salts are ammonia salts andeasily decomposed by heat.

The Met410 slurry was prepared as a 2× concentrate to facilitate thespray drying operation. This represents a slurry that is twice asconcentrated in each of the solids and dissolved solids as a slurrynormally used for polishing. The ingredients of this predrying slurrywere as follows:

Phosphoric acid: 11.2%  Ammonium Phosphate 8.5% Ammonium persulfate:8.5% Organic corrosion inhibitor 0.3% Titanium dioxide 9.9% D.I. waterBalance

The ingredients were mixed together and dispersed with a Hill rotorstator mixer until the temperature rise in the tank reached 10 degreesCelsius.

Within three days a portion of this material was spray dried in an APVLaboratory Spray Dryer having an electric air heater, a rotary atomizer,and a peristaltic feed pump with variable speed motor. See FIG. 1. Thedried powder was collected from the bottom of the dryer as well as thecyclone separator. The atomizer was adjusted to prevent product fromsticking on the walls of the dryer, and the inlet air temperature andfeed rate were adjusted to obtain an outlet air temp of 80-85 degreesCelsius. The powder was sealed in a plastic bag awaiting furtherprocessing. The remaining slurry was stored in a closed container as aliquid awaiting further testing. The details of this spray drying testare listed in Table 1 below:

TABLE 1 Inlet Temp. Outlet Temp. Feed Rate Atomizer Test (° C.) (° C.)(mL/min) speed(rpm) I 250-260 80-85 130 49000 II 180-185 80-85  65 41000

The two tests denote two different inlet temperatures at which the dryerwas operated to determine if inlet temperature had an effect on thedecomposition of volatile or heat sensitive components.

Another means of monitoring the usefulness of this invention is in theanalytical determination of the primary active ingredient in the slurry,the ammonium persulfate. This component is both sensitive to heat and tolight in the aqueous state. The two dried slurries and the concentratefrom which they came were analyzed one week after the spray drying wasperformed. The results are shown in Table 2 below. All concentrationswere made in the ready to use concentration.

From Table 2 it can be seen that the spray drying preserves the efficacyof the least stable chemical component. Even within ten days of itsmanufacture, the strength of the oxidizing component, ammoniumpersulfate, was decreasing within the single part slurry concentrate.

TABLE 2 median mean Slurry % persulfate pH particle size particle sizeReconstituted test I 4.34 2.3 0.178 0.233 Diluted liquid con- 4.17 2.30.177 0.225 Centrate, 10 d. old Reconstituted test II 4.33 2.3 0.1780.227

It can be seen from the data in Table 2 that the persulfateconcentration did not change with increased inlet temperature andremained exactly as it was intended in the original composition. Theparticle size of the two dried materials came back to exactly what theyhad been before the drying operation. The graphical depiction of thecomparison of particle size distributions of the predrying concentrate,and the two reconstituted test slurries is shown in FIG. 3. The threematerials are so close that the three distributions appear to be one.

Six weeks after spray drying, polishing tests were performed on thepre-drying slurry concentrate and the dried slurry sample Test I of theMet410 slurry. The final concentrations of these slurries were adjustedto the following levels:

Phosphoric acid: 5.6% Ammonium Phosphate 4.3% Ammonium persulfate: 4.3%Organic corrosion inhibitor  0.15% Titanium dioxide 4.9% D.I. waterBalance

The spray dried slurry was measured out to obtain the aboveconcentration and dispersed in a Hill rotor stator mixer to a delta T of10 degrees Celsius. The residual six week old concentrated slurry wasdiluted approximately 1 part concentrate to 1 part water to get theexact concentration noted above and mixed with a propeller mixer. Allslurries were filtered through a 1 micron bag filter.

Six inch diameter copper film wafers were polished with each of thethree slurries using a Strasbaugh 6EC Wafer Polisher at identicalpolishing parameters. Also polished were silicon dioxide film wafers toestablish the relative polishing rate between copper and silicon dioxideknown as the oxide selectivity. The results of these tests are shown inTable 3. The polishing rates are listed in Angstroms per minute (A/m):

TABLE 3 Slurry Origin Copper Removal Rate Selectivity to Oxide Predryingconcentrate, 2200 A/min 26:1 Cu:Oxide rate diluted to use conc. Spraydried reconstituted 7049 A/min 63:1 Cu:Oxide rate slurry # I

Typical removal rates for Met410 slurry are 6000 to 7000 A/min. The sixweek old concentrate had deteriorated considerably during that time as aone component mixture. The spray dried material had preserved thecharacteristics of the commercial two part slurry.

EXAMPLE 2

The second spray dried material known as Test II above was tested in acopper wafer polishing comparison nine weeks after it was dried. Thetest was run as a comparison of the reconstituted Test II material and afreshly mixed Met410 slurry. The polishing was performed in essentiallythe same manner as before. The concentrations of the two slurries weremade up to the end use concentration shown above. Results of thepolishing tests are shown in Table 4 below:

TABLE 4 Slurry Origin Copper Removal Rate Selectivity to Oxide FreshMet410 slurry 6100 A/min 36:1 Cu:Oxide rate Spray dried reconstituted6600 A/min 66:1 Cu:Oxide rate slurry # II

The surface finish of these wafers was also studied. There appeared tobe no noticeable difference in surface scratching between the twoslurries.

EXAMPLE 3

A concentrate identical to the pre-drying concentrate of examples 1 and2 was prepared in the same manner in preparation for another spray dryertest.

Within three days a portion of this material was spray dried in an NiroProduction Minor Spray Dryer having an electric air heater, a rotaryatomizer, and a Moyno feed pump with a variable speed motor. All of thedried powder was collected from the cyclone separator. The atomizer wasadjusted to prevent product from sticking on the walls of the dryer, andthe inlet air temperature and feed rate were adjusted to obtain anoutlet air temp of 100 degrees Celsius. The powder was sealed in aplastic bag awaiting further processing. The remaining slurry was storedin a closed container as a liquid awaiting further testing.

Several weeks later polishing tests were performed on the pre-dryingconcentrate slurry of example 3 and dried slurry samples as well as acommercial two part version of the Met410 slurry. The finalconcentrations of all three slurries were adjusted to the followinglevels:

Phosphoric acid: 5.6% Ammonium Phosphate 4.3% Ammonium persulfate: 4.3%Organic corrosion inhibitor  0.15% Titanium dioxide 4.9% D.I. waterBalance

The spray dried slurry was measured out to obtain the aboveconcentration and dispersed in a Hill rotor stator mixer to a delta T of10 degrees Celsius. The residual concentrated slurry from the spraydrying tests was diluted approximately 1 part concentrate to 1 partwater to get the exact concentration noted above and mixed with apropeller mixer. The commercial slurry was made by mixing Part A andPart B together (to obtain the above concentrations) and mixing with apropeller mixer. All slurries were filtered through a 1 micron bagfilter.

Six inch diameter copper film wafers were polished with each of thethree slurries using a Strasbaugh 6EC Wafer Polisher at identicalpolishing parameters. The results of these tests are shown in Table 5.

TABLE 5 Slurry Origin Copper Removal Rate Fresh Met410 slurry 6960 A/minSpray dried reconstituted slurry from 6333 A/min Niro Tests Predryingconcentrate, diluted to use 4994 A/min concentration (2 wks old)

Microscopic examination of these test wafers showed that the surfacecondition of the freshly made commercial slurry was better than thespray dried and reconstituted slurry in this case. The reason for thisdiscrepancy is unknown.

EXAMPLE 4

A Niro Mobile Minor Type H spray dryer was used in this example. Thisdryer was operated in a similar manner to the other examples. Thepolishing slurry in this example is used for polishing tungsten. Theproprietary slurry s a ready to use formulation had the followingconstituents:

Carboxylic acid 3% Carboxylic acid salt 0.3%   Oxidizing component 6%Abrasive 9% D.I. Water Balance

A concentrate of this slurry was prepared for spray drying by doublingthe concentrations of the chemical and abrasive components. Theconcentrate was then spray dried at several outlet temperature values todetermine the degradation, if any, of the carboxylic acid components.The outlet temperature was varied by changing the feed rate of theconcentrate and leaving all other parameters constant.

The dry flowable powders were reconstituted to the ready to use slurrycomposition shown above, and used to polish tungsten film wafers andthermal oxide film wafers. A portion of the original concentrate used tomake the powders was also diluted to the ready-to-use concentration andused as the baseline. Table 6 below shows the results of the spraydrying and polishing tests:

TABLE 6 Tungsten rate Selectivity to ox. Diluted concentrate from spray2344 A/min 416:1 drying run Freshly made slurry 2177 A/min 386:1Reconstituted spray dried 2328 A/min 346:1 powder

Microscopic examination of the surfaces of the oxide film wafers afterpolishing showed no difference in scratching between any of the testwafers.

Although the examples above report the use of this invention withchemical mechanical polishing slurries for integrated surfaceapplications, the value of this invention in other types of polishingformulations is obvious also. For example, many polishing compoundscontaining abrasives and chemicals, added to provide enhanced removalrates or special surface conditions, could be dried in a similar mannerand reconstituted at the point of use. Abrasive slurries such as thisare used for polishing specialty optics, for polishes involvingsemiconductor substrates such as silicon and gallium arsenide, forplastic eyeglass and contact lenses and other similar technologies.

What is claimed is:
 1. A dry polishing powder to be reconstituted by theaddition of water, wherein the dried polishing powder is produced bydrying a pre-dispersed aqueous polishing slurry having submicronabrasive particles dispersed therein.
 2. A powder according to claim 1,wherein the submicron abrasive particles are selected from the groupconsisting of alumina, silica, ceria, titania, and mixtures thereof. 3.A powder according to claim 2, wherein said submicron abrasive particlesare titania.
 4. A powder according to claim 1, wherein the slurryfurther comprises an oxidizing agent.
 5. A powder according to claim 1,wherein the slurry further comprises a chemical etchant.
 6. A powderaccording to claim 1, wherein the slurry further comprises a dispersingagent.
 7. A powder according to claim 1, wherein the slurry furthercomprises a complexing agent.
 8. A powder according to claim 1, whereinthe slurry further comprises a corrosion inhibitor.